Composition and method for polishing a composite of silica and silicon nitride

ABSTRACT

A method is provided for polishing a composite comprised of silica and silicon nitride wherein a polishing composition is used comprising: an aqueous medium, abrasive particles, a surfactant, an organic polymer viscosity modifier which increases the viscosity of the composition, and a compound which complexes with the silica and silicon nitride wherein the complexing agent has two or more functional groups each having a dissociable proton, the functional groups being the same or different.

This application is a continuation of application Ser. No. 09/071,566filed May 1, 1998, now U.S. Pat. No. 6,132,637 which is aContinuation-in-Part of and incorporates by reference prior applicationSer. No. 09/037,668 filed Mar. 10, 1998 now U.S. Pat. No. 6,042,741which is a Divisional Application of 08/802,829 filed Feb. 19, 1997 nowU.S. Pat. No. 5,738,800 which claims the benefit of U.S. ProvisionalApplication No. 60/027,277 filed Sep. 27, 1996.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to compositions which are useful asslurries for the chemical-mechanical polishing of substrates, especiallysubstrates comprised of silica and silicon nitride. More specificallythe slurries of the present invention include an aqueous medium,abrasive particles, a viscosity modifier, a surfactant, and a compoundwhich complexes with silica and silicon nitride.

2. Description of Related Art

In integrated circuit technology, various active and passive elementsgenerally must be isolated from one another within the integratedcircuit structure. This has often been accomplished by deep or shallowtrench isolation techniques. These techniques typically use silicondioxide (silica) as a dielectric material and silicon nitride as a stoplayer, requiring chemical-mechanical polishing (planarization) of eachcircuit layer. To achieve efficient polishing and adequateplanarization, a polishing slurry is generally useful and should providea high selectivity involving the removal rate of silica relative tosilicon nitride.

In Silvestri et al., U.S. Pat. No. 4,526,631, a slurry of 6 weightpercent colloidal silica adjusted to a pH of about 12 with KOH providesa polishing ratio of about 10 SiO₂ to 1 Si₃N₄. Beyer et al., U.S. Pat.No. 4,671,851, states that the polishing ratios between SiO₂ and Si₃N₄preferably should be between a lower limit of 4 to 1 and a higher limitof 40 to 1. Beyer describes obtaining a ratio of 6.2 to 1 using acolloidal silica in water with small amounts of a sodium salt ofdichloroisocyanuric acid and sodium carbonate.

Even more recent patents such as Murase, U.S. Pat. No. 5,502,007, alsodescribe obtaining selectivities of about 10 SiO₂ to 1 Si₃N₄ removalrates using a colloidal silica slurry as a polishing agent. Kodera etal., U.S. Pat. No. 5,445,996, use ceria as well as silica for theabrasive particles in slurries, but they also report selectivities forSiO₂ to Si₃N₄ removal rates in the range of 2 to 3.

SUMMARY OF THE INVENTION

A composition is provided for polishing a composite comprised of silicaand silicon nitride comprising: an aqueous medium, abrasive particles, asurfactant, a viscosity modifier, and a compound which complexes withthe silica and silicon nitride wherein the complexing agent has two ormore functional groups each having a dissociable proton, the functionalgroups being the same or different.

A further aspect of the invention is the method for polishing acomposite comprised of silica and silicon nitride comprising: applying aslurry at a polishing interface between a polishing pad and thecomposite comprised of silica and silicon nitride, the slurrycomprising: an aqueous medium, abrasive particles, a surfactant, aviscosity modifier, and a compound which complexes with the silica andsilicon nitride wherein the complexing agent has two or more functionalgroups each having a dissociable proton, the functional groups being thesame or different.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

It has been found that the addition of a compound which complexes withsilica and silicon nitride to polishing slurries used in thechemical-mechanical polishing of composites comprised of silica andsilicon nitride can provide a very high selectivity of rate of removalof SiO₂ to the rate of removal of Si₃N₄ when a surfactant is used inconjunction with the complexing agent and when the concentration of thecomplexing agent in the slurry is sufficient to block the removal ofSi₃N₄ while not greatly affecting the removal of SiO₂ at the pH of thepolishing slurry.

Compounds which act as complexing agents or chelating agents for SiO₂and Si₃N₄ are described in great detail in U.S. Pat. Nos. 5,391,258 and5,476,606 which are made part of this specification by reference. Thesecompounds must have at least two acid groups present in the structurewhich can affect complexation to the silica and silicon nitride surface.Acid species are defined as those functional groups having a dissociableproton. These include, but are not limited to, carboxylate, hydroxyl,sulfonic and phosphonic groups. Carboxylate and hydroxyl groups arepreferred as these are present in the widest variety of effectivespecies. Particularly effective are structures which possess two or morecarboxylate groups with hydroxyl groups in an alpha position, such asstraight chain mono- and di-carboxylic acids and salts including, forexample, malic acid and malates, tartaric acid and tartarates andgluconic acid and gluconates. Also effective are tri- and polycarboxylicacids and salts with secondary or tertiary hydroxyl groups in an alphaposition relative to a carboxylic group such as citric acid andcitrates. Also effective are compounds containing a benzene ring such asortho di- and polyhydroxybenzoic acids and acid salts, phthalic acid andacid salts, pyrocatecol, pyrogallol, gallic acid and gallates and tannicacid and tannates. In the examples to follow, a salt of phthalic acid isused as the complexing agent and, therefore, such salts are preferredcomplexing agents for this invention. Potassium hydrogen phthalate,“KHP”, was the phthalate salt used in the experiments described below.

The surfactant used in conjunction with the complexing agent in thisinvention is not present to perform the usual function of surfactants inslurries of stabilizing the particulate dispersion. As shown in theexamples which follow, the surfactant in combination with the complexingagent affects the rate of removal of Si₃N₄ from the composite surface.It is believed that any surfactant, whether it be an anionic, cationic,non-ionic or zwitter-ionic surfactant, might be effective in thecompositions of this invention. Particularly useful would befluorocarbons or hydrocarbons with phosphate end groups. In thefollowing examples several different surfactants were shown to beeffective. “ZFSP”, ZONYL™ FSP Fluorosurfactant, manufactured by theDuPont Company, was shown to be a particularly effective surfactantadditive to the slurries of this invention. It is a long straight chainhydrocarbon with phosphate groups at one end and a fluoride at the otherend.

In these examples, ceria was used for the abrasive particles in theslurry because it is an effective polishing abrasive forchemical-mechanical polishing at all pH conditions and is stable againstgelation. Any other polishing abrasive, such as alumina, zirconia,silica, titania and barium carbonate could also be used.

To adjust the slurries of this invention to the pH range in which thehighest selectivities of SiO₂ removal to Si₃N₄ removal are obtained anybase or amine compound might be used. In the examples to follow KOH isused to adjust the pH of the slurry compositions. Potassium hydroxide,ammonium hydroxide, and all types of soluble amine compounds may be usedto adjust the pH of chemical-mechanical polishing slurries.

To inhibit scratching, a viscosity modifier is preferably added to theslurry compositions of the present invention. The viscosity modifier isintended to increase the polishing fluid boundary layer between thepolishing pad and substrate being polished. Generally speaking, theheight of the boundary layer is proportional to the square root of theviscosity. Therefore, if the viscosity is increased four fold, theboundary layer should approximately double. As the boundary layerincreases, the opportunity for pad-to-substrate contact decreases,thereby diminishing the opportunity for unwanted scratching. A largerboundary layer will generally provide improved lubrication and heattransfer at the polishing interface and will generally provide moreefficient slurry transport along the polishing interface. By increasingthe polishing fluid viscosity and thereby also increasing the boundarylayer, slurry particles have less opportunity to drag across a substratesurface, but rather, the slurry particles will tend to be acceleratedthrough contact with the pad and cause the particle to impinge uponprotrusions from the polishing substrate, thereby creating fluid basedwear.

Useful viscosity enhancing agents include hydrophilic, water swellablecrosslinked polymers, such as polyacrylic acid and poly vinyl acetate.Inorganic viscosity enhancing agents are also well known and includecertain clays and ceramic particles which will increase the viscosity ofwater. A preferred viscosity modifier is ethylene glycol polymer andcopolymer.

EXAMPLE 1

Table 1 shows the results of polishing silicon dioxide and siliconnitride wafers with slurries containing various amounts of complexingagent at selected pH levels. These experiments were carried out on aStrasbaugh 6DS SP Planarizer using an IC1000/SubaIV polishing pad stackunder the conditions of 7 psi down pressure, 1.5 psi back pressure, 30rpm carrier speed and 32 rpm table speed with a slurry flow rate of 125ml/min. 6 inch wafers were used and the pad was conditioned after eachwafer was polished. All slurries in this series of experiments contained0.45% colloidal ceria and 0.2% ZFSP surfactant and the pH of the slurrywas adjusted using potassium hydroxide.

TABLE 1 Sample % KHP* pH RR SiO₂ RR Si₃N₄ Selectivity 1 0 4 1419 256 5.52 3.1 4 6 3 2 3 0.5 7 3019 189 16 4 1 7 3000 15 200 5 3.1 7 1185 4 296 61 10 3397 994 3.5 7 2 10 3246 920 3.5 *Potassium hydrogen phthalate

These results clearly show that selectivities of silicon dioxide tosilicon nitride removal rates much higher than reported heretofore canbe obtained by using slurries comprising a complexing agent and asurfactant at a pH level where these additives effectively shut down theremoval rate of silicon nitride while not greatly affecting the removalrate of silicon dioxide. As shown above, selectivities of 200 andgreater can be obtained by this inventive method of polishing.

EXAMPLE 2

The following experiments show the necessity of using a surfactant inthe slurries of this invention. They were carried out on a Strasbaugh6DS SP Planarizer under the same conditions described in Example I.Slurries were similar to the one used for Sample 4 on Table I in thatthe slurries were comprised of 1% KHP, 0.45% colloidal ceria and the pHwas adjusted to 7 using KOH. Percent surfactant, ZFSP, was either 0.2%as in Sample 4 or 0.0%. Removal rates for Si₃N₄ in Angstroms per minuteare shown in Table 2 below:

TABLE 2 % ZFSP RR Si₃N₄ 0 793 0 768 0 736 0.2 10 0.2 8

It is obvious from these data that the surfactant is critical to theshutting down of the Si₃N₄ removal rate so that such extraordinaryselectivities can be obtained.

EXAMPLE 3

In the following experiment the abrasive in the slurries was commercialopaline ceria which was milled before use. Polishing of wafers wasaccomplished under the same conditions as in Example 1. Results aregiven in Table 3 below:

TABLE 3 % % % RR RR Sample Abrasive KHP ZFSP pH SiO₂ Si₃N₄ Selectivity 8 2 0 0 7 5804 3504   2  9 2 1 0.2 7 2642 22 120 10 3.5 1 0.2 6.5 319513 246 11 5 1 0.2 6.5 3705 33 112 12 3.5 2 0.4 7 2123 10 212 13 5 2 0.47.5 3609 1105   3

These results show that a complexing agent and a surfactant effectivelyshut down the removal rate of Si₃N₄ when used in a slurry adjusted to adesired pH range. At a pH of 6.5 and 7 the removal rate of Si₃N₄ isgreatly shut down giving selectivities for the removal of silica overthat of silicon nitride of over 100 (samples 9-12). However, at a pH of7.5 (sample 13), the removal rate of silicon nitride is no longerdepressed and selectivities are very poor.

EXAMPLE 4

In these experiments several surfactants were found to be effective inreducing the removal rate of silicon nitride at a pH of 6.5. Thesurfactants used were FC-93, Fluorad™ FC-93, an anionic fluorochemicalsurfactant available from the 3M Company; “PVS”, the sodium salt ofpolyvinyl sulphonate available commercially; and “ZFSN”, ZONYL™ FSN, anon-ionic surfactant available from the DuPont Company. The slurries inthis Example all contained 1.5% KHP (potassium hydrogen phthalate) and0.45% commercial opalene ceria as the abrasive. Polishing of wafers wasaccomplished under the same conditions as in Example 1. Results aregiven in Table 4 below:

TABLE 4 Sample Surfactant % Surfactant RR SiO₂ RR Si₃N₄ Selectivity 14FC-93 0.2 2975 464 6 15 PVS 0.3 3406 35 98 16 ZFSN 0.3 2678 39 68

These results show that several surfactants are effective in slurriesmade to reduce the removal rate of silicon nitride. A given surfactantmight be even more effective should pH and slurry compositions beoptimized.

EXAMPLE 5

In this Example the abrasive used was WS2000 available from Rodel, Inc.WS2000 is an abrasive which contains both ceria and silica. The slurryused for this experiment contained 3.5% abrasive, 1.5% KHP (potassiumhydrogen phthalate), and 0.2% ZFSP (ZONYL™ FSP). The pH was about 6.5.Results from the polishing of wafers under the same conditions as inExample 1 are shown on Table 5 below:

TABLE 5 Sample RR SiO₂ RR Si₃N₄ Selectivity 17 2209 9 244

The above Examples show many embodiments of this invention and are notmeant to restrict the invention in any way. The scope of the inventionis defined only by the claims which follow:

What is claimed is:
 1. A method of polishing a composite comprised ofsilica and silicon nitride comprising: applying a slurry at a polishinginterface between a polishing pad and said composite comprised of silicaand silicon nitride, said slurry comprising: an aqueous medium, abrasiveparticles, a surfactant, an organic polymer viscosity modifier whichincreases the viscosity of the slurry, and a compound which complexeswith said silica and silicon nitride, wherein said compound has two ormore functional groups each having a dissociable proton, the functionalgroups being the same or different wherein the Ph of said composition isadjusted to a range wherein the removal rate of silicon nitride iseffectively shut down while the removal rate of silicon dioxide is notgreatly affected.
 2. A method according to claim 1 wherein said compoundwhich complexes with silica and silicon nitride contains a benzene ring.3. A method according to claim 1 wherein said compound which complexeswith silica and silicon nitride is a straight chain mono- ordi-carboxylic acid or salt which has a secondary hydroxyl group in analpha position relative to a carboxylate group.
 4. A method according toclaim 1 wherein said compound which complexes with silica and siliconnitride is a tri- or poly-carboxylic acid or salt which has a secondaryor tertiary hydroxyl group in an alpha position relative to a carboxlategroup.
 5. A method according to claim 1 wherein said abrasive particlescomprise ceria.
 6. A method according to claim 1 wherein said surfactantcomprises a fluorosurfactant.
 7. A method according to claim 2 whereinsaid compound which complexes with silica and silicon nitride ispotassium hydrogen phthalate.
 8. A method according to claim 2 whereinsaid polishing composition comprises: water, about 0.2% to about 5%ceria, about 0.5% to about 3.5% potassium hydrogen phthalate, about 0.1%to about 0.5% fluorosurfactant, all percentages by weight, and whereinthe pH of said polishing composition is adjusted from about 6 to about 7by the addition of a base or an amine compound to said polishingcomposition.